The present invention relates to a control system for the drive of a power take-off mechanism on an agricultural vehicle, and, particularly, a tractor having a CVT transmission and a control for reading machine specific values of an implement attached to the tractor.
The drive of a pto on a tractor, which in principle coincides with such drives on all the other agricultural vehicles, is at present constructed in such a way that a gear box is connected to its engine for producing the speeds of 540, 750, 1000 or 1400 rpm which are usual in practice. Its drive shaft is connected to the input shaft by means of a hydraulically actuated clutch, to which pressure is admitted from the hydraulic circuit of the tractor via a solenoid valve, This drive train continues through the output shaft of the hydraulically actuated clutch, on which a brake of any desired type is arranged, up to the pto of the tractor.
At one end of the pto a drive shaft is attached, which is connected at its other end so as to drive the pto stub of an implement attached to the lift which is attached either at the front or the rear of the tractor. The control system for previously known drives for a pto is orientated exclusively towards the optimal design of acceleration and deceleration of the pto through the engagement and disengagement of the hydraulically actuated clutch, which is achieved by varying the quantity and with it the pressure build-up up or decay in the hydraulic fluid passed into the hydraulically actuated clutch. After the acceleration, control is no longer envisaged, so that then the speed of the pto in operation is directly proportional to the speed of the engine on the tractor.
For such control systems a multitude of variant embodiments has already become known, one of which is elaborated in WO 99/56978 A1. There a control device is fixed on the tractor, which contains a processor, which runs a special control program in accordance with the input signals passed to it and produces appropriate output signals. In so-called normal operation, the input signals come each from a pushbutton for switching the pto on and off and from a sensor for monitoring the engine speed and from a further sensor for monitoring the speed of the pto. Optionally the operator can switch over from a so-called HTS (headland turn sequence) to management operation, where further input signals can be sent to the control device via a pushbutton for switching on HTS or management operation and via sensors for monitoring the speed of the rear axle and the lift status (control and transport).
The output signals are passed on to a solenoid valve functioning as a switching valve, which in this case is a pulse-width modulated ON/OFF valve. This solenoid valve then passes the adjacent pressure of the hydraulic fluid from the hydraulic circuit of the tractor to the hydraulically actuated clutch, so that this engages and disengages smoothly over a definite period of time in accordance with a special control program from the control device. One part of the input signals ensures the smooth occurrence of these functions and another part in management operation controls automatically in addition the engagement and disengagement respectively of the hydraulically actuated clutch in accordance with the lift status of the tractor and its travelling speed, for which additional safety circuits are installed for the protection of the operator.
Detrimental to this control system is that it is designed for optimal engagement or disengagement respectively, by which is understood as rapid as possible and smooth engagement for avoiding wear and tear on the hydraulically actuated clutch, the transmission of high torques harmlessly through the clutch and for the avoidance of breakage of shear pins on the implement, important conditions of use were not taken into account. These include on the one hand changes in the viscosity of the hydraulic fluid through varying external temperatures. On the other hand no influence on the engagement process was accepted by the special control program in the control device, when at the moment of acceleration widely differing implement are employed. Likewise the initial speeds of the engine at the moment of acceleration or deceleration of the pto were ignored in the control program, although it is known that they can fluctuate markedly in consequence of the differing power taken from the engine.
A further disadvantage in this control system is seen, in that no facility for a manually variable adjustment for the smoothness of engagement and disengagement is envisaged. Therefore in this variant implementation it is a question of only a quite normal control mechanism, which cannot react by far so variably as a normal control system. All the stated disadvantages are the reason that the hydraulically actuated clutch thus controlled, the remaining drive elements and the implement under these conditions exhibit only a limited service life, as acceleration proceeds too abruptly. Furthermore, it is still to be criticised in this control mechanism, that in HTS or management operation respectively no influence was exerted on the engagement and disengagement of the hydraulically actuated clutch in relation to the desired lift height of the implement, which would however have been entirely desirable, given the variability of the implements that may be employed.
As a result on the one hand, damage to the drive shaft can occur, if this is not stopped in good time before a high angle of flexion is attained when the lift is raised, or if already in the presence of a high angle of flexion it is switched on again too soon. On the other hand in the converse case, when lowering the lift the drive shaft is not switched on again in good time, lost time occurs, which diminishes the effective time of use of the tractor, and at the same time the risk exists that in that event the engine stalls, as the implement is already in contact with the ground before the pto turns.
A further variant of a control system for accelerating and decelerating a pto on a tractor was published in EP 0 443 325 A1, its principal objective is to avoid, for the hydraulically actuated clutch and for the subsequent drive elements up to the implement under critical operating conditions, increased wear and tear and in the extreme case destruction of all the drive elements and the implements. The construction of this control system which is likewise working only as a control mechanism, coincides fully with solution described in the preceding paragraphs, as it likewise possesses a control device which picks up an input signal and from it forms an output signal, in which the output signals are passed on to a solenoid valve functioning as a switching valve for engaging or disengaging the hydraulically actuated clutch in the drive train to the drive shaft. In a preferred embodiment the solenoid is a proportional valve, which is driven by timed pulses from the control device and is thereby actuated.
As input signal, on the one hand the engine speed is monitored, in which the hydraulically actuated clutch is disengaged by the control device in the event of a fall in the engine speed below a certain preset limit, and when this speed is exceeded it is engaged again. As a further input signal the speeds of the input shaft and of the output shaft of the hydraulically actuated clutch are picked up by means of sensors, from the differences in which the control device calculates the slip that exists. If this exceeds a preset limit, then the hydraulically actuated clutch is disengaged and when the slip falls below this value again it is engaged again. In the same way torsional vibrations in the input shaft and the output shaft of the hydraulically actuated clutch are processed as input signals. Finally the height of lift of the lifting gear on the tractor is followed, and when a prescribed limit is reached it is disengaged.
For the purpose of achieving engagement that is as rapid and smooth as possible, a characteristic curve is stored in the control device, the function of which is comparable with the control program of the control mechanism in accordance with WO 99/56978 A1. As regards the disadvantages of this control mechanism, to avoid recapitulation of the disadvantages of the first described control mechanism, it is pointed out that these are identical. However a further disadvantage arises in addition for this control mechanism, as in it no HTS or management operation is provided for. In addition, both of the variant embodiments described in the foregoing have quite significant common disadvantages. Thus acceleration and deceleration of the pto is mostly unsatisfactory, as the hydraulically actuated clutch permits only short wear times.
To improve this situation, therefore, very complicated and hence cost-intensive control systems are employed. The necessity for the presence of a clutch, a gearbox with long gear levers and a brake leads necessarily to a large construction, which results in disadvantages in the design of the driver's cab. In addition the clutch is an expendable part that wears out and incurs costs. But also after acceleration of the pto, quite significant disadvantages arise for its drive through the lack of a control system in this area. Thus through the direct driving connection between the engine of the tractor and the pto with interposition of the gearbox and the hydraulically actuated clutch a continuously variable selection of the pto speed independently of the engine speed is not possible.
This means that the engine must be driven constantly in a high-speed range, so that the fuel consumption of the engine is high and the driver is thereby subjected to unnecessary annoyance through noise. In addition, it is still to be criticised that the PTO MODE to be found in some agricultural vehicles, by which is to be understood keeping the engine speed constant at all driving speeds, experiences limitations, as at a constant engine speed the driving transmission ratio can be altered only very slowly and hence the vehicle becomes sluggish.
In the case, which occurs not seldom in practice, that the speed of the engine drops also the maximum speed of the pto is limited. This circumstance leads in the case of implements that require a very constant pto speed, that these are switched off from the drive automatically. A very typical example of this would be the use of a sowing machine. Finally, with such a drive the mode of working of the pto as an accurate ground speed pto cannot be realised. A very important disadvantage of this drive is also that reversal of the direction of rotation of the pto is not possible.